This invention relates, in general, to pressure sensing circuits, and more particularly to pressure sensing circuits for measuring low pressures.
A common semiconductor pressure sensor, such as the MPX2010 sensor manufactured by Motorola Inc., uses a resistor bridge to generate a differential voltage. The pressure sensor is biased with a D.C. voltage. The magnitude of the voltage determines the sensitivity and range of the pressure sensor. A port on the pressure sensor exposes a diaphragm which modifies resistance values in the resistor bridge when a pressure is applied The pressure sensor outputs a differential voltage which is proportional to the pressure at the port.
Measuring low pressures accurately can be a problem since the magnitude of the pressure sensor differential voltage output is reduced at the lower pressures. This is compounded by the fact that small pressure differences will produce minute incremental voltage changes at the pressure sensor outputs which may be extremely difficult or impossible to resolve. There are pressure sensing circuits which attempt to overcome these problems but yield mixed results.
One technique commonly used is to amplify the differential output to a level where it can be easily measured or manipulated. A high gain amplification stage (gain greater than 500) must be used for low pressure applications. An amplifier used in a high gain configuration may have stability problems or nonlinearities which affect accuracy. Poor signal to noise ratios may result due to the small signal levels.
A second technique which produces better results is to bias the pressure sensor at a higher D.C. voltage. The higher bias voltage will increase the differential output voltage at a given pressure, thus incremental pressure changes will also result in larger voltage changes from the pressure sensor. A lower gain amplification stage can be used resulting in a higher signal to noise ratio and a more stable system. The drawback to this approach is that the bias voltage on the pressure sensor can only be increased so much. Power dissipation and other reliability/accuracy factors must be taken into account at the higher bias voltages.
It would be of great benefit if a pressure sensing circuit or method therefor could be developed which further increases sensitivity to allow accurate measurement of even lower pressures.